Universal Mobile Telecommunication System (“UMTS”) is a world wide system known as an evolution of the Global System for Mobile Communications (“GSM”; originally Groupe Special Mobile) and a mobile communication terminal for use in the UMTS is generally referred to as a UE (User Equipment).
The UMTS is a mobile cellular network comprising a set of base stations BS(i) and user equipments UE(j). The base stations communicate with user equipments through wireless links, using electromagnetic waves referred to as RL (Radio Links).
In a normal mode, a User Equipment UE usually communicates with a single network base BS through two radio links, a downlink DL and an uplink UL. The downlink DL is the radio link RL from the base station BS to the user equipment UE which carries individual channels, for example a DPCH (Dedicated Physical Channel) containing a logical channel DCH (Dedicated Channel) assigned to the user equipment UE.
Generally, the propagation range of a radio link determines a coverage area or cell Ci for each base station BS(i) considered as a node of the cellular network. While being located at the boundary of a currant cell, let C1, and moving from C1 to a at least one adjacent cell Cad (j), the user equipment UE maintains without any interruption a communication call by using simultaneously the same logical channel DCH sent from each adjacent cell base station BSad(i) through their own DPCH(i) whereby each DPCH(j) is different from each other. This mode of operation is called a Soft Handover (SHO). The recombination of the information coming from each cell is done at the symbol level.
It is well known that frequency offset that occurs in a mobile communication system causes unavoidable performance deterioration. Frequency offset may be generated by the frequency drift of oscillators with temperature and/or Doppler effect on propagation waves. An AFC (Automatic Frequency Control) operation for compensating for the frequency offset is therefore needed. Many receivers with an automatic frequency control function have been already described extensively for radio mobile physical channel under various propagation conditions as determined by Rayleigh Rice and/or multipath signals. In all cases, the radio link RL, herein using a DPCH, is considered as a distinct, fully characterized signal.
The objective problem is that there exists no receiver with an AFC function that operates in soft handover mode and also efficiently compensates the offset frequencies of the various radio links in order to demodulate the unique logical channel DCH in a simple way and with high performance.